Sports rackets used in such sports as tennis, squash, and badminton are made from a generally oval frame of laminated wood, metal or composite materials and include an integral handle or grip. The open area within the oval frame is covered with criss-crossing strings, usually made from nylon. The pattern of criss-crossing is usually vertical columns (main strings) and horizontal rows, with respect to the handle. Although multi-string lengths (one length of string for each vertical column and horizontal row) have been used with some rackets, the rackets available today generally consist of a single length, or two lengths of nylon string (i.e, one length for the columns and one for the rows). U.S. Pat. No. 4,484,742 describes a multi-string racket. The separate lengths of string are laced through appropriate apertures located along the perimeter of the racket frame.
The procedure for stringing rackets using a single length string requires the use of a stringing machine having a frame brace attachment which prevents the frame from warping during the tensioning of the strings. The prior art procedure for stringing the rackets mounted in the stringing machine begins with the main (vertical) strings followed by the installation of the horizontal strings. The brace prevents warping or structural damage of the frame due to the uneven tension of strings. For example, tensioning only the main strings will pull the frame longitudinally and will probably warp it or break it without the frame brace.
The machines currently used to string sports rackets and apply the precise tension to each row and column of the string are generally large, heavy, immobile machines and require trained personnel for their operation.
U.S. Pat. No. 4,484,742 offers an alternative hand-held stringing machine that can be used without the need for clamping the racket frame, however the hand held device disclosed in U.S. Pat. No. 4,484,742 is designed only for use with multi-length strings. The device disclosed in U.S. Pat. No. 4,484,742 includes the use of a cone-shaped anchor positioned at each open end of each string length (two per column lengths and row lengths). The anchor is designed to compress about the string at a point adjacent the frame. The force of compression of each anchor is provided by the tension residing in the string length after tensioning. The result with the device and system disclosed in U.S. Pat. No. 4,484,742 is that each string length can only get more taut during tensioning. The anchors act as one-way clamps, only allowing string movement away from the center of the frame.
The system disclosed by U.S. Pat. No. 4,484,742 is not desirable for use with today's sports rackets for several reasons including the added weight factor gained by attaching the many required anchors along the periphery of the frame, the complexity in forming the racket frame with the mating cone-shaped recesses to accommodate the anchors. Other deficiencies of the system disclosed in U.S. Pat. No. 4,484,742 include the additional amount of time needed to insert each anchor and tension each string length, the undesirable trauma exerted to the string at each end by the clamping contact of each anchor, and the fact that if a particular string is over-tightened, it must be cut and replaced with a new one.
It is important that today's sports rackets remain lightweight, simple in construction and assembly and include a frame structure which can maintain proper string tension without exerting trauma to the strings. If excessive trauma is applied, the strings will break, either during tensioning or during use. The device shown in U.S. Pat. No. 4,484,742 cannot be used for tensioning any racket that does not include the above-mentioned anchors because without the anchors, the device cannot accommodate the additional length of the string caused by natural stretching of nylon. Nylon, for example, can stretch up to 40% its original length before retaining a tension of 55 lbs. The device of U.S. Pat. No. 4,484,742 cannot draw and hold the string to proper string tension without the use of the anchors because, apart from the anchors, there is no retaining means for maintaining a certain tension to a length of string while tightening it to a higher tension.
In other words, the device of U.S. Pat. No. 4,484,742 can only pull the string in predetermined increments, which are too small to overcome the increased length of the string due to stretching. The device must therefore rely on the clamping action of the otherwise undesirable anchors.
It is therefore an object of the present invention to provide a portable compact string tensioning device for use with sports rackets having a one or two length string design and that do not require clamping anchors along the periphery of the frame.
It is another object of the present invention to provide the portable stringing device with means for accommodating the additional length of nylon string generated during the tensioning procedure due to the natural stretching property of nylon string.
It is another object of the present invention to provide a method of stringing a racket frame with the present apparatus which does not require the use of a frame warp-preventing brace.
It is another object of the present invention to provide a portable and adjustable frame support bar to prevent frame warpage during traditional string tensioning procedures.
It is another object of the present invention to provide a portable stringing device which is both simple to use and inexpensive to manufacture.